598 



MOLLUSCA 



PHYLUM VI 



known only in tlieir later stages of development. Sipliiincle of moderate size, sub- 

 ventran. Funnels reaching half-way across each camera, steeply inclined orad, and 



split on the outer side. Closure of the walls effected by a 

 plate extending from the apical opening of each funnel through 

 the funnel itself orad to the apical opening of the next 

 beyond, and projecting into the interior as a flattened fold, 

 which is incomplete or open along the central axis. These 

 internal collars or flat semiconical rings have been described 

 as complete cones (Dwight). Ordovician. 



Suborder D. ORTHOCHOANITES Hyatt. 



Gerontic stages have uncontracted volutions and open aper- 

 tures, except in a few uncoiled pliyloyerontic genera. Siphun- 

 FiG. 1108. cular segments may be slightly niimmuloidal, fusiform or tubular, 



Conoceras irracpost^rwm but are never markedly nummuloidal, nor are the funnels bent 

 D)"vosdf'^Boh(fmia^fafter s/i-ar^^^ outwards as in Cyrtochoanites. Deposits formed only in 

 Barrande). the siphuncles of Orthoceratidae and Kionoceras, and in them 



they are irregular and no endosiphuncles occur ; other genera 

 have empty siphuncles. Funnels, as a rule, both longer and straighter than in Cyrto- 

 choanites, and in Atu,ria almost equal to those of Holochoanites. 



Tliis group includes the greater number of Nautiloid forms, passing from the 

 smoothest to the most highly ornamented of Paleozoic shells, continuing in the Trias 

 as nautilicones of comjilex ornamentation, and terminating with smooth shells that 

 range from the Jura to the present time. The sutures become more sinuous and com- 

 plex in one of the subdivisions than in all other Nautiloids. The increase in number 

 of lobes and saddles begins in the Trias with Clymenonautilus, and ends with Aturia 

 in the Tertiary. 



I. Orthoceratida Hyatt. 



Orthoceracones and cyrtoceracones with smooth or ornamented shells, and not as a 

 rule contracted in gerontic stage ; apertures open throughout life. Although often short, 

 none are brcvicones, strictly speaking. Section circular or elliptical, very rarely oval. 

 Siphuncle with slightly nummidoidal, fusiform or tubular segments, and generally near 

 the centre. 



Family 1. Orthoceratidae M'Coy. 



Section circular or compressed, living chamber uncontracted or only slightly so, 

 and aperture always open. Surface smooth or with only transverse bands, rarely 

 longitudinal striae, never longitudinal ridges. Siphuncle small (except in Baltoceras), 

 segments fusiform or cylindrical, never nummuloidal. Deposits when present irregular, 

 and gathered about the funnels as in the Cyrtochoanites; no definite endosiphuncles ever 

 formed. 



Baltoceras Holm. Sii^huncle large, but with short, straight funnels, and sheaths 

 as in Orthoceras. Ordovician. 



Orthoceras Breyn (Figs. 1109, 1110). Long tapering orthoceracones and cyrto- 

 ceracones, smooth, or with only transverse striae and growth bands. Siphuncle 

 generally larger than in Geisonoceras, centren or slightly dorsad of centre. Deposits 

 when present gathered about the funnels as in the Annulosiphonata. Silurian to Trias, 



Geisonoceras Hyatt (Fig. 1111). Similar to the last, but sides spreading more 

 rapidly, and sii>huncle empty, centren or slightly ventrad of centre. Ordovician to 

 Carboniferous. 



